The transverse electro-optic (EO) modulation system is built based on cubic boron nitride (cBN) single crystals unintentionally doped and synthesized at a high pressure and high temperature. The photoelectric output of the system includes two parts that can be measured respectively and the value of elements in the linear EO tensor of the cBN crystal can be obtained. This method does not need to measure the absolute light intensity. All of the surfaces of the tiny cBN crystals whose hardness is next to the hardest diamonds are {111} planes. The rectangular parallelepiped cBN samples are obtained by cleaving along {110} planes and subsequently grinding and polishing {112} planes of the tiny octahedral cBN flakes. Three identical non-zero elements of the EO tensor of the cBN crystal are measured via two sample configurations, and the measured results are very close, about 3.68 and 3.95 pm/V, respectively, which are larger than the linear EO coefficients of the general III-V compounds.OCIS codes: 160.2100, 160.4760, 190.4400, 190.4720. doi: 10.3788/COL201210.041602. Cubic boron nitride (cBN) has a zinc blend structure and belongs to the T d point group; thus, it possesses the linear electro-optic (EO) effect. Its EO tensor possesses only three identical non-zero elements, namely, γ 41 = γ 52 = γ 63 . The bandgap of cBN is about 6.3 eV [1−3] , which is the widest among the III-V compounds. Therefore, cBN is transparent in throughout the visible range and most of the infrared and ultraviolet spectra; it can also operate in the wide spectral range as the EO material. Furthermore, cBN has a very high laser damage threshold, meaning it can withstand high-power pulsed or continuous-wave (CW) laser beams. Thus, cBN has potential EO applications. The determination of the EO coefficients of cBN crystals is a prerequisite not only for the EO applications, but also for other studies (e.g., for the study of Raman efficiency of cBN [4] and for the propagation control of the surface wave generated from the interface between the cBN crystal and other isotropic medium [5] ). Some different theoretical results of the EO tensor of the cBN crystals have been reported [6−8] , but these results were only evaluated based on the secondorder non-linear susceptibility χ (2) (−2ω, ω, ω), which is related to the second harmonic generation. In fact, the linear EO coefficient γ 41 (ω) should be obtained based on χ (2) (−ω, ω, 0), which is relevant to the linear EO effect and optical rectification. Because of the frequency dispersion of the second-order non-linear susceptibility, χ (2) (−2ω, ω, ω) = χ (2) (−ω, ω, 0); that is, the accurate value of γ 41 (ω) should come from the experiments of the linear EO effect or optical rectification. Unfortunately, very few reports [9] have yet examined the experimental value of γ 41 of cBN crystals. The main obstacles may include the difficulties of synthesizing the high-quality large cBN crystals and fabricating cBN crystals into suitable rectangular parallelepiped samples because of the second highe...